(1) Technical Field
The present invention relates to an exothermic conducting paste or coating and an electric resistance heating unit, particularly to an exothermic conducting paste for providing an electric resistance heating unit which generates an uniform temperature distribution at any temperature and has the temperature self-controlling property, and an electric resistance heating unit which is arbitrarily adjustable to a desired temperature below 350.degree. C.
(2) Background Information
Japanese Patent Publication No. 60-59131/1985 discloses a planar electric heating element comprising a synthetic resin band having conductive fine powder such as carbon black or graphite incorporated therein and electrode wires buried in the band at both ends in the longitudinal direction thereof. The temperature of this element can be increased to about 60.degree. C. A heating unit comprising a solid lined with this element is also known.
However, the carbon black or graphite powder is high in electric specific resistance (5,000 to 20,000 .mu..OMEGA.cm) and negative in temperature coefficient of electric resistance (about -2.6 .mu..OMEGA.cm/.degree.C.). Accordingly, for the heating unit containing such a conductive fine powder, the distance between electrodes on a coated film is narrow, for example, and a large heating surface having an uniform temperature distribution can not be obtained. In the heating unit wherein the conductive fine powder such as carbon black or the like is used, there is utilized the tape-shaped heating element which is formed by melt extrusion from the synthetic resin having this conductive fine powder incorporated therein. Heating units having a large heating surface are seldom prepared by the use of a paste or paint containing such a conductive fine powder.
Since the conventional heating unit was in danger of local oxidation or damage by burning, the temperature of this unit could only be increased to a temperature below about 60.degree. C.
For example, in the conventional heating unit, a substrate 1 is lined with a planar heating element (tape) 2 as shown in FIGS. 7a and 7c. When electricity is supplied through metal terminals 3, a heating part 7 is heated and a temperature distribution 4 as shown in FIG. 7b developed.
Thus, the conventional conductive powder such as carbon black or the like is high in electric specific resistance and negative in temperature coefficient of electric resistance. Accordingly, for the heating unit containing such a conductive powder, the distance between electrodes on the coated film, the tape or the like can not be widen and the large heating surface having an uniform temperature distribution can not be obtained. When the substrate is coated with the paste or coating containing such a conductive powder, the thickness of the coated film must be precisely controlled. It is further necessary that the paste or coating be applied by a machine, for example, to a thickness of not more than 0.3 mm.+-.0.02 mm, and it is unsuitable that the paste or coating be manually applied. According to the conventional heating unit, more electric current is supplied to a thicker portion on the variation of the thickness of the coated film, and consequently the temperature of that portion is elevated. However, the decrease of electric resistance results in flowing of progressively more electric current, because the conventional conductive fine powder such as carbon black or the like is negative in temperature coefficient of electric resistance. Accordingly, the temperature of that portion becomes still higher, and local damage by melting or by burning is induced thereby.
Further, according to the prior art, the curved surface, the inner surface of the hole or the uneven surface is impossible to be precisely coated therewith by means of the machine. Therefore, the coated film having an uniform thickness can not be obtained and the local heating as described above undesirably takes place. In the conventional planar heating elements, the curved surface, the inner surface of the hole or the uneven surface is difficult to be lined with the element tape, and the width of the element tape is necessarily narrowed because of their high resistance. When applied on the large area, a number of these tapes are used. As a result, the temperature difference occurs between the tapes and the heating part, and accordingly, it is impossible to heat the whole of the wide surface at an uniform temperature. Further, this heating element is only heated to a temperature of about 60.degree. C. and can not be adjusted to a higher desired temperature.
Therefore, there has long been a need for an exothermic conducting paste or coating for providing a heating unit with a large heating surface on which an uniform temperature distribution can be obtained, even if a substrate has a complex structure such as a curved surface, the inner surface of the hole or an uneven surface, and the substrate is coated with the paste or coating to a thickness not so precisely uniform by hand or by impregnation, the local damage by melting or by burning does not take place, and the heating temperature can be freely controlled.